Previously, many features have been provided on inflatable vehicle occupant restraint systems for controlling inflation and deflation of an air bag. Some of these devices incorporate porous fabric panels in the construction of an air bag, while others incorporate gas generating inflators which tailor delivery of inflation gases into an air bag. Alternatively, or in combination, air vents are provided in air bags for expelling inflation gases from within an inflating or deflating air bag.
Frequently, vents have been provided in air bags to expel excess exhaust gases from within a bag which can cause over pressurization or failure of the bag as a result of an occupant impact with the bag during a vehicle collision. Typically, excess inflation gases are provided within an air bag in order to enhance the speed with which an air bag is inflated in order to assure the full deployment an air bag in a timely manner for presentation to a forward moving occupant during a vehicle crash. However, these excess gases must be vented from within the bag subsequent to full deployment in order to present a compliant air bag to the occupant which has a reduced stiffness during impact. Moreover, most air bag vents which provide variable air flow are designed to restrict the venting of gases during inflation and before an occupant impacts the air bag, and increase gas venting once the occupant impacts the bag.
U.S. Pat. No. 3,820,814 discloses a discharge valve for an air bag which provides a more tailored discharge of exhaust gases through an aperture. A cup-shaped housing is provided in the walls of the air bag which has an aperture for expelling exhaust gases. A flexible cover is supported upstream of the aperture by a plurality of elastic bands which vary position of the cover with respect to the aperture in order to flow restricting exhaust gases which exit through the aperture. In order to stably support and present the cover with the elastic bands, the housing must provide a rigid or semi-rigid support structure or else presentation of the cover with respect to the aperture cannot be accurately controlled and tailored. However, the incorporation of a rigid or semi-rigid housing on the surface of an air bag is a problem because a hard-spot is formed by the housing on the air bag surface which might abrade or injure an occupant, and could further contribute to occupant head injury. Contact of an occupant's head with a rigid or semi-rigid object is not desirable, and incorporation of such a valve on an air bag would require placing the valve at locations where the occupant does not make contact with the bag.
U.S. Pat. No. 5,246,250 discloses a tether-actuated valve assembly provided in an air bag which pulls a gas impermeable valve flap across a pair of flow regulating panels in order to block the flow of gas from an air bag as it approaches a full state of inflation. However, such a flow regulating device is bulky and complicated in operation, and might be unreliable. Furthermore, such a device reduces flow from an air bag which tends to restrict flow as the bag reaches a fully inflated state. It is preferable to provide a device which tailors expulsion of inflation gases from an air bag based on pressure within the bag. For example, such a desired air bag would decrease the exhausting of inflation gases from a bag when the bag exceeds a threshold pressure due to impact by a large occupant or a high energy impact resulting from a high speed collision in order to prevent bottoming out of the air bag by an occupant.